Multi-input DC/DC converters with zero-current switching

ABSTRACT

A modular-based, zero-current-switched (ZCS), isolated full-bridge boost converter with multiple inputs is disclosed. Each converter module is used to match the connected input source and control the amount of power drawn from the source. The power from various sources are combined together and delivered to the load through a multiphase transformer. The input inductor of each boost-derived converter module keeps the input current constant and acts as a current source to drive the multiphase transformer through a phase-shifted-controlled full bridge converter. By connecting an auxiliary circuit across the full-bridge input in each module, the transformer leakage inductance and output capacitance of the switching devices are used to create resonant paths for facilitating zero-current-switching of all switching devices.

FIELD

The present disclosure relates to multi-input (and single-input) DC/DC converters with zero-current-switching.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Various DC/DC converters are known in the art for supplying dc power to loads, including multi-input DC/DC converters that can combine energy from multiple dc power input sources. Many of these converters incorporate full bridge converters having switches. Ideally, these switches would be turned on or off under zero-current conditions (so-called zero-current-switching) to minimize switching losses. Many existing converters are, however, incapable of achieving zero-current-switching of the main switches during both switching transitions. As a result, such converters suffer from switching losses that reduce the overall efficiency of the DC/DC converter.

SUMMARY

According to one example of the present disclosure, a multi-input dc/dc converter includes a plurality of input modules, a transformer, at least one output terminal for supplying dc power to a load, and a rectifier circuit. Each input module includes at least one input terminal for coupling to a dc power source and a full bridge converter having a plurality of main switches. The transformer has a plurality of primary windings and at least one secondary winding. Each primary winding is coupled to one of the full bridge converters. The rectifier circuit is coupled between the secondary winding of the transformer and the output terminal. Each input module further includes an auxiliary circuit coupled to the full bridge converter for achieving substantially zero-current-switching of the main switches during turn-on and turn-off of the main switches.

According to another example of the present disclosure, a multi-input DC/DC converter includes a plurality of input modules, a transformer having a plurality of primary windings and at least one secondary winding, at least one output terminal for supplying dc power to a load, and a rectifier circuit coupled between the secondary winding of the transformer and said output terminal. Each input module includes at least one input terminal for coupling to a dc power source, a full bridge converter having a plurality of main switches, and means for achieving substantially zero-current-switching of the main switches during turn-on and turn-off of the main switches. Each primary winding of the transformer is coupled to one of the full bridge converters.

According to a further example of the present disclosure, a DC/DC converter includes at least one input terminal for coupling to a dc power source, a full bridge converter having a plurality of main switches, a transformer having at least one primary winding coupled to the full bridge converter and at least one secondary winding, at least one output terminal for supplying dc power to a load, a rectifier circuit coupled between the secondary winding of the transformer and the output terminal, and an auxiliary circuit coupled between the input terminal and the full bridge converter for achieving substantially zero-current-switching of the main switches during turn-on and turn-off of the main switches.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a block diagram of a multi-input DC/DC converter according to one example of the present disclosure.

FIG. 2 is a block diagram of each input module shown in FIG. 1.

FIG. 3 is a circuit diagram of the ith input module shown in FIG. 1 according to one specific embodiment of this disclosure.

FIG. 4 is a circuit diagram of a two-input DC/DC converter according to another example of the present disclosure.

FIGS. 5( a)-5(l) illustrate the twelve operating stages of each input module shown in FIG. 1 and constructed according to the circuit diagram of FIG. 3.

FIG. 6 is a timing diagram for each input module shown in FIG. 1 and constructed according to the circuit diagram of FIG. 3.

FIG. 7 illustrates the waveform of i_(o,i) for determining a voltage conversion ratio.

FIG. 8 illustrates a control circuit for the auxiliary circuit shown in FIG. 3 according to another example of the present disclosure.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

A multi-input DC/DC converter according to one example of the present disclosure is illustrated in FIG. 1 and indicated generally by reference number 100. As shown in FIG. 1, the converter 100 includes N input modules 102, 104, 106, 108, a transformer 110, a rectifier circuit 112, and output terminals 114 for supplying dc power to a load (N represents a positive integer greater than 1). Each input module 102-108 includes input terminals for coupling to a dc power source. Additionally, each input module 102-108 includes output terminals coupled to a primary winding n₁-n_(N) of the transformer 110. The rectifier circuit 112 is coupled between a secondary winding n₀ of the transformer 110 and the converter output terminals 114, as shown in FIG. 1. In this manner, power from various dc power sources are combined together and delivered to the load through the transformer 110.

In the example of FIG. 1, each input module has the same general circuit configuration, which is illustrated in FIG. 2. Accordingly, the example of FIG. 1 is a modular-based multi-input DC/DC converter. As shown in FIG. 2, each input module includes an input choke L_(i), an auxiliary circuit 220, and a full bridge converter 230. The auxiliary circuit 220 is coupled between the input choke L_(i) and the full bridge converter 230. As further explained below, the auxiliary circuit is configured for achieving substantially zero current switching (ZCS) (also referred to as “soft-switching”) of the converter's main switches during turn-on and turn-off of such switches. The auxiliary circuit essentially converts the hard-switched isolated boost converter (the main switches of the full bridge converter) into a soft-switched isolated boost converter. The auxiliary circuit does this without significantly affecting the macro-operation of the multi-input converter 100. As a result, the converter of FIG. 1 has reduced losses and thus improved efficiency (on the order of 6-8%) as compared to prior art multi-input DC/DC converters.

FIG. 3 illustrates one specific embodiment of the auxiliary circuit and full bridge converter shown in FIG. 2. More specifically, FIG. 3 illustrates the equivalent circuit for a specific embodiment of the ith input module 106 shown in FIG. 1. In the embodiment of FIG. 3, the auxiliary circuit 220 includes an auxiliary inductor L_(r,i), an auxiliary capacitor C_(r,i) and an auxiliary switch S_(Au,i) coupled in series. The auxiliary switch includes a body diode D_(Au,i) that inhibits current flow from the input choke L_(i) when the auxiliary switch is off. Further, the full bridge converter 230 is a phase-shift-controlled full bridge converter having four main MOSFET switches S_(1,i)-S_(4,i) each configured for unidirectional operation. In the specific embodiment of FIG. 3, the main switches are configured for unidirectional operation by coupling the switch in series with blocking diodes D_(1,i)-D_(4,i).

The auxiliary circuit forms a quasi-resonant circuit—with the transformer leakage inductance and the output capacitances of main switches—that facilitates zero-current-switching of the main switches in the full bridge converter. During the stages that power is transferred to the output side of the converter, the resonant time is only three quarters of the resonant period. Therefore, the auxiliary circuit is called a quasi-resonant circuit.

The low side switches S_(3,i)-S_(4,i) of the full-bridge converter achieve zero-current switching during turn-off due to the resonance of the auxiliary capacitor and the auxiliary inductor. The current starts from zero, resonates to a peak value and returns to a level equal to the input current. The current then remains at such level until the power transferring stage ends. The low side switches also achieve zero-current switching during turn-on due to the auxiliary capacitor, the auxiliary inductor and the leakage inductance of the transformer. The high side switches S_(1,i)-S_(2,i) of the full-bridge converter also turn-on and turn-off with zero current due to the transformer winding current reset. Accordingly, the switching losses in the full-bridge converter are quite low.

Further, the auxiliary switch achieves zero-current switching during turn-on due to the resonance of the auxiliary capacitor and the auxiliary inductor. The auxiliary switch also achieves zero-voltage switching during turn-off due to the resonance of the auxiliary capacitor, the auxiliary inductor and the leakage inductance of the transformer. Thus, soft-switching of the auxiliary switch can be achieved during both switching transitions (turn-on and turn-off). Accordingly, the switching losses in the auxiliary circuit are also quite low.

One example of a suitable phase-shift controller for each full-bridge converter is the UC3875 integrated circuit available from Texas Instruments Incorporated. This circuit includes a bi-directional synchronization (CLOCKSYNC) pin. This pin of several ICs (one for each full-bridge converter) can be connected together so that the gate signal will be synchronized by the IC with the fastest local oscillating frequency. In this manner, the gate signals of the high side switches are synchronized for all modules. The phase shift of the low side switches is determined by the feedback circuit of the phase-shift controller.

Each auxiliary circuit can be controlled by a control circuit that operates independently in each module. In other words, the control circuits for the auxiliary circuits need not be synchronized. Each control circuit can include a current transformer for sensing the current in the auxiliary circuit. The turn-on time of the auxiliary switch is pre-set depending on the auxiliary capacitor and the auxiliary inductor values. The auxiliary switch can be turned-off any time that its body diode is conducting.

Although FIG. 1 illustrates a DC/DC converter with four input modules, it should be understood that the DC/DC converter can have more or less input modules without departing from the teachings of this disclosure. For example, FIG. 4 illustrates a multi-input DC/DC converter having only two input modules. Further, while multi-input DC/DC converters are described above, it should be understood that the teachings of present disclosure also apply to single input DC/DC converters (having only one dc power source). For example, the circuit illustrated in FIG. 3 can be used by itself as a single input DC/DC converter (having only one input module i).

The operating principles for one specific embodiment of the converter of FIG. 1—where each input module includes an auxiliary circuit and a full-bridge converter as shown in FIG. 3—will now be described. Each module goes through twelve operating stages in one switching cycle. These operating stages are described further below, and are illustrated in FIGS. 5A-5L. FIG. 6 illustrates the timing diagram for each module. The switches S_(1,i) and S_(2,i) are synchronized with the other modules, while the switches S_(3,i) and S_(4,i) are independently controlled for determining the input-to-output voltage requirement. The auxiliary circuit formed by the switch S_(Au,i), resonant inductor L_(r,i), and resonant capacitor C_(r,i) is used to provide ZCS conditions for all switches.

To simplify the analysis below, the following assumptions have been made: (i) the transistors and diodes are perfect; they have zero on-state resistance, infinite off-state resistance, and zero junction capacitance; (ii) all the energy storage components are free of loss without parasitic element; and (iii) the operations of the switch pair S_(1,i) and S_(2,i), and the switch pair S_(3,i) and S_(4,i), are similar; the magnitude of the output voltage is controlled by adjusting the phase shift between the two switch pairs.

Stage 1 [t₀-t₁] [FIG. 5( a)]: Transformer Current Reset Stage

A closed path formed by the transformer, transformer leakage inductance L_(l,i), S_(1,i), and S_(2,i) is created. The output voltage V_(out) reflected to the transformer primary side is applied across L_(l,i) and makes the transformer primary current decrease. S_(2,i) is then turned off with zero current at the end of this stage. The input inductor L_(i) is charged by the supply source v_(S,i). The equations of this stage are

$\begin{matrix} {{i_{S,i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 12},i}}},{i_{{Cr},i} = 0},\; {i_{{S\; 1},i} = {\left( \frac{{v_{S,i}L_{l,i}} + {v_{P,i}L_{i}}}{L_{i}L_{l,i}} \right)t}},{i_{{S\; 2},i} = {{- \left( {\frac{v_{P,i}}{L_{l,i}}t} \right)} + I_{{L\; 12},i}}}} & \; \\ {{i_{{S\; 3},i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 12},i}}},{i_{{S\; 4},i} = 0}} & (2) \\ {{v_{{Cr},i} = V_{{C\; 0},i}},{v_{{S\; 1},i} = {v_{{S\; 2},i} = {v_{{S\; 3},i} = 0}}}} & (3) \end{matrix}$

where I_(L12,i) is the input current at the end of the last cycle,

$v_{P,i} = {\frac{n_{i}}{n_{o}}V_{out}}$

is the output voltage reflected to the primary side of the transformer, and V_(C0,i) is the voltage across C_(r,i) at the end of stage 12 and is also equal to the value at the end of stage 11.

The duration of this stage is Δt_(1,i)=t₁−t₀. The gate signals that apply to the two high side switches S_(1,i) and S_(2,i) are overlapped. The duration of this stage is Δt_(1,i)=t₁−t₀. The gate signals that apply to the two high side switches S_(1,i) and S_(2,i) are overlapped

2) Stage 2 [t₁-t₂] [FIG. 5( b)]: Inductor Charging Stage

L_(i) is continually charged by v_(S,i) through the switches S_(1,i) and S_(3,i). The equations of this stage are

$\begin{matrix} {{i_{S,i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 12},i}}},{i_{{S\; 1},i} = {i_{{S\; 3},i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 1},i}}}},{i_{{Cr},i} = 0},{i_{{S\; 2},i} = {i_{{S\; 4},i} = 0}}} & (4) \\ {{v_{{Cr},i} = V_{{C\; 0},i}},{v_{{S\; 1},i} = {v_{{S\; 3},i} = 0}}} & (5) \end{matrix}$

where I_(L1,i) is the value of i_(S,i) at t₁.

The duration of this stage is Δt_(2,i). It determines the average current through L_(i) and the magnitude of V_(out).

3) Stage 3 [t₂-t₃] [FIG. 5( c)]: Resonant Stage I

S_(Au,i) and S_(4,i) are turned on with zero current. C_(r,i), L_(r,i), S_(1,i), and S_(3,i) form a resonant path. The energy stored in C_(r,i) will generate a resonant current that will make the current through S_(1,i) and S_(3,i) zero. The stage ends when S_(3,i) is turned off with zero current and zero voltage. i_(Cr,i) will reach to the value equal to i_(S,i). The equations of this stage are

$\begin{matrix} {{i_{S,i} = {{{\frac{v_{S,i}}{L_{i}}t} + {I_{{L\; 2},i}i_{{S\; 1},i}}} = {i_{{S\; 3},i} = {{\frac{v_{S,i}}{L_{i}}t} + {\frac{V_{{C\; 0},i}}{Z_{{o\; 3},i}}\sin \; \omega_{3,i}t} + I_{{L\; 2},i}}}}},{i_{{S\; 2},i} = {i_{{S\; 4},i} = 0}}} & \; \\ {i_{{Cr},i} = {{- \frac{V_{{C\; 0},i}}{Z_{{o\; 3},i}}}\sin \; \omega_{3,i}t}} & (6) \\ {{v_{{Cr},i} = {V_{{C\; 0},i}\cos \; \omega_{3,i}t}},{v_{{S\; 1},i} = {v_{{S\; 3},i} = {v_{{S\; 4},i} = 0}}}} & (7) \\ {{{where}\mspace{14mu} I_{{L\; 2},i}\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {value}\mspace{14mu} {of}\mspace{14mu} i_{S,i}\mspace{14mu} {at}\mspace{14mu} t_{2}},{\omega_{3,i} = \frac{1}{\sqrt{L_{r,i}C_{r,i}}}},{{{and}\mspace{14mu} Z_{{o\; 3},i}} = {\sqrt{\frac{L_{r,i}}{C_{r,i}}}.}}} & \; \end{matrix}$

The duration of this stage, Δt_(3,i)=t₃−t₂, is determined by solving (5) and (6) that

$\begin{matrix} {{\frac{- V_{{C\; 0},i}}{Z_{{o\; 3},i}}\sin \; \omega_{3,i}\Delta \; t_{3,i}} = {{\frac{v_{S,i}}{L_{i}}\Delta \; t_{3,i}} + I_{{L\; 2},i}}} & (8) \end{matrix}$

The voltage of v_(Cr,i) at t₃ is determined by solving (6) and (7),

v _(Cr,i)(t ₃)=V _(C1,i) =V _(C0,i) cos ω_(3,i) Δt _(3,i)   (9)

4) Stage 4 [t₃-t₄] [FIG. 5( d)]: Capacitor Constant Current Charging Stage

As the duration of this stage is short and the input current has small variation, L_(i) is considered as a constant current in this stage. The junction capacitance of S_(3,i), C_(S3,i), and C_(r,i) undergo constant current charging. This stage ends when the two capacitor voltages increase to the level equal to the output voltage reflected to the primary side of the transformer. The equations of this stage are

$\begin{matrix} {{{i_{S,i} = {{\frac{v_{S,i} - V_{{C\; 1},i}}{Z_{{o\; 4},i}}\sin \; \omega_{4,i}t} + {I_{{L\; 3},i}\cos \; \omega_{4,i}t}}},\begin{matrix} {i_{{Cr},i} = i_{{SAu},i}} \\ {= {\frac{C_{r,i}}{C_{r,i} + C_{{S\; 3},i}}\left\lbrack {{\frac{v_{S,i} - V_{{C\; 1},i}}{Z_{{o\; 4},i}}\sin \; \omega_{4,i}t} + {I_{{L\; 3},i}\cos \; \omega_{4,i}t}} \right\rbrack}} \\ {{\approx i_{S,i}},} \end{matrix}}\begin{matrix} {i_{{S\; 3},i} = {\frac{C_{{S\; 3},i}}{C_{r,i} + C_{{S\; 3},i}}\left\lbrack {{\frac{v_{S,i} - V_{{C\; 1},i}}{Z_{{o\; 4},i}}\sin \; \omega_{4,i}t} + {I_{{L\; 3},i}\cos \; \omega_{4,i}t}} \right\rbrack}} \\ {{\approx 0},} \end{matrix}\begin{matrix} {i_{{S\; 1},i} = i_{{S\; 2},i}} \\ {= i_{{S\; 4},i}} \\ {= 0} \end{matrix}} & (10) \\ {\begin{matrix} {v_{{Cr},i} = {v_{S,i} - {\left( {v_{S,i} - V_{{C\; 1},i}} \right)\cos \; \omega_{4,i}t} + {Z_{{o\; 4},i}I_{{L\; 3},i}\sin \; \omega_{4,i}t}}} \\ {{\approx {{\frac{i_{S,i}}{C_{r,i} + C_{{S\; 3},i}}t} + V_{{C\; 1},i}}},} \end{matrix}{v_{{S\; 1},i} = {v_{{S\; 4},i} = 0}}} & (11) \end{matrix}$

where I_(L3,i) is the input current at t₃,

${\omega_{4,i} = \frac{1}{\sqrt{L_{i}\left( {C_{r,i} + C_{{S\; 3},i}} \right)}}},{{{and}\mspace{14mu} Z_{{o\; 4},i}} = {\sqrt{\frac{L_{i}}{C_{r,i} + C_{{S\; 3},i}}}.}}$

The duration of this stage, Δt_(4,i)=t₄−t₃, is obtained by using (10) that

v _(P,i) −v _(S,i)=−(v _(S,i) −V _(C1,i))cos ω_(4,i) Δt _(4,i) +Z _(o4,i) I _(L3,i) sin ω_(4,i) Δt _(4,i)   (12)

5) Stage 5 [t₄-t₅] [FIG. 5( e)]: Resonant Stage II

S_(4,i) starts conducting. C_(r,i), L_(r,i), S_(1,i), S_(4,i), the output capacitances of S_(2,i) and S_(3,i), C_(S2,i) and C_(S3,i), and the leakage inductance of the transformer L_(l,i) form a resonant circuit. S_(Au,i) will be turned off with zero current and zero voltage, when the diode D_(Au,i) conducts. This stage ends when D_(Au,i) turns off with zero current.

$\begin{matrix} {{{i_{S,i} = {{\frac{\left( {v_{S,i} - v_{P,i}} \right)C_{r,i}L_{l,i}^{2}}{\left( {L_{i} + L_{l,i}} \right)\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{5,i}t} + {\frac{L_{l,i}I_{{L\; 4},i}}{L_{i} + L_{l,i}}\cos \; \omega_{5,i}t} - {\frac{v_{P,i} - v_{S,i}}{L_{i} + L_{l,i}}t} + \frac{L_{i}I_{{L\; 4},i}}{L_{i} + L_{l,i}}}},\mspace{20mu} {i_{{Cr},i} = {{\frac{\left( {v_{S,i} - v_{P,i}} \right)L_{l,i}C_{r,i}}{\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{5,i}t} + {I_{{L\; 4},i}\cos \; \omega_{5,i}t}}},\begin{matrix} {\mspace{20mu} {i_{{S\; 1},i} = i_{{S\; 4},i}}} \\ {= {{\frac{\left( {v_{P,i} - v_{S,i}} \right)C_{r,i}L_{i}L_{l,i}}{\left( {L_{i} + L_{l,i}} \right)\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{5,i}t} -}} \\ {{{{\frac{L_{i}I_{{L\; 4},i}}{L_{i} + L_{l,i}}\cos \; \omega_{5,i}t} - {\frac{v_{P,i} - v_{S,i}}{L_{i} + L_{l,i}}t} + \frac{L_{i}I_{{L\; 4},i}}{L_{i} + L_{l,i}}},}} \end{matrix}}\mspace{20mu} {i_{{S\; 2},i} = {i_{{S\; 3},i} = 0}}} & (13) \\ {{v_{{Cr},i} = {{I_{{L\; 4},i}Z_{{o\; 5},i}\sin \; \omega_{5,i}t} - {\frac{\left( {v_{S,i} - v_{P,i}} \right)L_{l,i}}{L_{i} + L_{l,i}}\cos \; \omega_{5,i}t} + \frac{{L_{i}v_{P,i}} + {L_{l,i}v_{S,i}}}{L_{i} + L_{l,i}}}},\mspace{20mu} {v_{{S\; 1},i} = {v_{{S\; 4},i} = 0}},\begin{matrix} {\mspace{20mu} {v_{{S\; 2},i} = v_{{S\; 3},i}}} \\ {= {{\frac{\left( {v_{P,i} - v_{S,i}} \right)L_{i}L_{l,i}^{2}}{\left( {L_{i} + L_{l,i}} \right)\Phi_{i}}\cos \; \omega_{5,i}t} +}} \\ {{{\frac{L_{i}L_{l,i}I_{{L\; 4},i}}{\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{5,i}t} + \frac{{v_{S,i}L_{l,i}} + {v_{P,i}L_{i}}}{L_{i} + L_{l,i}}}} \end{matrix}} & (14) \end{matrix}$

where I_(L4,i) is the input current at t₄,

${\omega_{5,i} = \sqrt{\frac{L_{i} + L_{l,i}}{C_{r,i}\Phi_{i}}}},{Z_{{o\; 5},i} = \sqrt{\frac{\Phi_{i}}{C_{r,i}\left( {L_{i} + L_{l,i}} \right)}}}$

and Φ_(i)=L_(r,i) L_(i)+L_(r,i) L_(l,i)+L_(i) L_(l,j).

The duration of this stage Δt_(5,i) is obtained by solving i_(Cr,i)=0in (13) that

$\begin{matrix} {{\Delta \; t_{5,i}} = {{t_{5} - t_{4}} = {{\frac{1}{\omega_{4,i}}\tan^{- 1}\frac{I_{{L\; 4},i}\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}{\left( {v_{P,i} - v_{S,i}} \right)L_{l,i}C_{r,i}}} \approx {\frac{3\pi}{2}\frac{1}{\omega_{5,i}}}}}} & (15) \end{matrix}$

At the end of this stage, v_(Cr,i)=V_(C5,i) and is determined by using (14) and (15).

$\begin{matrix} {{v_{{Cr},i}\left( t_{5} \right)} = {V_{{C\; 0},i} = {{{- I_{{L\; 4},i}}Z_{{o\; 5},i}} + \frac{{L_{i}v_{P,i}} + {L_{l,i}v_{S,i}}}{L_{i} + L_{l,i}}}}} & (16) \end{matrix}$

The maximum voltage and current rating of the main switches, v_(S2(max),i), v_(S3(max),i), i_(S1(max),i), and i_(S4(max),i), can be obtained by solving (13) and (14), respectively.

$\begin{matrix} {{v_{{S\; 2{(\max)}},i} = {v_{{S\; 3{(\max)}},i} = {\frac{L_{i}L_{l,i}I_{{L\; 4},i}}{\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}} + \frac{{v_{S,i}L_{l,i}} + {v_{P,i}L_{i}}}{L_{i} + L_{l,i}}}}}{and}} & (17) \\ \begin{matrix} {i_{{S\; 1{(\max)}},i} = i_{{S\; 4{(\max)}},i}} \\ {= {\frac{\left( {v_{P,i} - v_{S,i}} \right)C_{r,i}L_{i}L_{l,i}}{\left( {L_{i} + L_{l,i}} \right)\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}} -}} \\ {{{\frac{v_{P,i} - v_{S,i}}{L_{i} + L_{l,i}}\left( \frac{\pi}{2\omega_{5,i}} \right)} + {\frac{L_{i}}{L_{i} + L_{l,i}}I_{{L\; 4},i}}}} \end{matrix} & (18) \end{matrix}$

6) Stage 6 [t₅-t₆] [FIG. 5( f)]: Inductor Discharging Stage

The energy stored in L_(i) releases to the output together with the input at a relatively constant current. This stage ends when S_(2,i) is turned on with zero current.

$\begin{matrix} {{i_{S,i} = {{\frac{v_{S,i} - v_{P,i}}{L_{i}}t} + I_{{L\; 5},i}}},{i_{{Cr},i} = 0},{i_{{S\; 1},i} = {i_{{S\; 4},i} = {{\frac{v_{S,i} - v_{P,i}}{L_{i}}t} + I_{{L\; 5},i}}}},{i_{{S\; 2},i} = {i_{{S\; 3},i} = 0}}} & (19) \\ {{v_{{Cr},i} = V_{{C\; 0},i}},{v_{{S\; 1},i} = {v_{{S\; 4},i} = 0}}} & (20) \end{matrix}$

where I_(L5,i) is the input current at t₅.

The duration of this stage Δt_(6,i) is equal to

$\begin{matrix} {{\Delta \; t_{6,i}} = {{t_{6} - t_{5}} = {\left( {\theta_{i}\frac{T_{S}}{2}} \right) - \left( {\frac{3\pi}{2}\frac{1}{\omega_{5,i}}} \right)}}} & (21) \end{matrix}$

where θ_(i)=t₆−t₄.

As shown in FIG. 6, θ_(i) is approximately equal to the phase shift between the switch pairs of S_(1,i)-S_(2,i) and S_(3,i)-S_(4,i), as the duration of stage 4 is short.

7) Stage 7 [t₆-t₇] [FIG. 5( g)]: Transformer Current Reset Stage

This stage is similar to stage 1. The reflected voltage of V_(out) on the primary side is applied across L_(l,I) through S_(1,i) and S_(2,i). This makes the transformer primary current decrease linearly. S_(1,i) is turned off with zero current before the end of this stage. The input inductor L_(i) is charged by the supply source v_(S,i). The equations of this stage are

$\begin{matrix} {{{i_{S,i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 6},i}}},{i_{{Cr},i} = 0},{i_{{S\; 1},i} = {{{- \frac{v_{P,i}}{L_{l,i}}}t} + I_{{L\; 6},i}}},{i_{{S\; 2},i} = {\frac{{v_{S,i}L_{l,i}} + {v_{P,i}L_{i}}}{L_{i}L_{l,i}}t}}}{{i_{{S\; 3},i} = 0},{i_{{S\; 4},i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 6},i}}}}} & (22) \\ {{v_{{Cr},i} = V_{{C\; 0},i}},{v_{{S\; 1},i} = {v_{{S\; 2},i} = {v_{{S\; 4},i} = 0}}}} & (23) \end{matrix}$

where I_(L6,i) is the input current at t₆.

The duration of this stage is Δt_(7,i)=t₇−t₆. The gate signals that apply to the two high side switches S_(1,i) and S_(2,i) are overlapped.

8) Stage 8 [t₇-t₈] [FIG. 5( h)]: Inductor Charging Stage

L_(i) is continually charged by v_(S,i) through the switches S_(2,i) and S_(4,i). The equations of this stage are

$\begin{matrix} {{i_{S,i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 7},i}}},{i_{{S\; 1},i} = {i_{{S\; 3},i} = 0}},{i_{{S\; 2},i} = {i_{{S\; 4},i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 7},i}}}},{i_{{Cr},i} = 0}} & (24) \\ {{v_{{Cr},i} = V_{{C\; 0},i}},{v_{{S\; 1},i} = {v_{{S\; 3},i} = 0}}} & (25) \end{matrix}$

where I_(L7,i) is the input current at t₇.

The duration of this stage is Δt_(8,i). Similar to stage 2, it determines the average current through L_(i) and the magnitude of V_(out).

9) Stage 9 [t₈-t₉] [FIG. 5( i)]: Resonant Stage I

S_(Au,i) and S_(3,i) are turned on with zero current. C_(r,i), L_(r,i), S_(2,i), and S_(4,i) form a resonant path. The energy stored in C_(r,i) will generate a resonant current that will make the current through S_(2,i) and S_(4,i) zero. The stage ends when S_(4,i) is turned off with zero current and zero voltage. i_(Cr,i) will reach to the value equal to i_(S,i). The equations of this stage are

$\begin{matrix} {{{i_{S,i} = {{\frac{v_{S,i}}{L_{i}}t} + I_{{L\; 8},i}}},{i_{{S\; 1},i} = {i_{{S\; 3},i} = 0}},{i_{{S\; 2},i} = {i_{{S\; 4},i} = {{\frac{v_{S,i}}{L_{i}}t} + {\frac{V_{{C\; 0},i}}{Z_{{o\; 9},i}}\sin \; \omega_{9,i}t} + I_{{L\; 8},i}}}}}{i_{{Cr},i} = {{- \frac{V_{{C\; 0},i}}{Z_{{o\; 9},i}}}\sin \; \omega_{9,i}t}}} & (26) \\ {{v_{{Cr},i} = {V_{{C\; 0},i}\cos \; \omega_{9,i}t}},{v_{{S\; 2},i} = {v_{{S\; 3},i} = {v_{{S\; 4},i} = 0}}}} & (27) \end{matrix}$

where I_(L8,i) is the value of i_(S,i) at t₁, ω_(9,i)=ω_(3,i) and Z_(o9,i)=Z_(o3,i).

The duration of this stage, Δt_(9,i)=t₉−t₈, is determined by solving (26) and (27) that

$\begin{matrix} {{\frac{- V_{{C\; 0},i}}{Z_{{o\; 9},i}}\sin \; \omega_{9,i}\Delta \; t_{9,i}} = {{\frac{v_{S,i}}{L_{i}}\Delta \; t_{9,i}} + I_{{L\; 8},i}}} & (28) \end{matrix}$

The value of Δt_(9,i) is equal to Δt_(3,i).

10) Stage 10 [t₉-t₁₀] [FIG. 5( j)]: Resonant Capacitor Constant Current Charging Stage

Similar to Stage 4, the junction capacitance of S_(4,i), C_(S4,i), and C_(r,i) undergo constant current charging. This stage ends when the capacitor voltages increase to the level equal to the output voltage reflected to the primary side of the transformer. The equations of this stage are

$\begin{matrix} {{{i_{S,i} = {{\frac{v_{S,i} - V_{{C\; 1},i}}{Z_{{o\; 10},i}}\sin \; \omega_{10,i}t} + {I_{{L\; 9},i}\cos \; \omega_{10,i}t}}},\begin{matrix} {i_{{Cr},i} = i_{{SAu},i}} \\ {= {\frac{C_{r,i}}{C_{r,i} + C_{{S\; 4},i}}\left\lbrack {{\frac{v_{S,i} - V_{{C\; 1},i}}{Z_{{o\; 10},i}}\sin \; \omega_{10,i}t} + {I_{{L\; 9},i}\cos \; \omega_{10,i}t}} \right\rbrack}} \\ {{\approx i_{S,i}},} \end{matrix}}{{i_{{S\; 1},i} = {i_{{S\; 2},i} = {i_{{S\; 3},i} = 0}}},{i_{{S\; 4},i} = {{\frac{C_{{S\; 4},i}}{C_{r,i} + C_{{S\; 4},i}}\left\lbrack {{\frac{v_{S,i} - V_{{C\; 1},i}}{Z_{{o\; 10},i}}\sin \; \omega_{10,i}t} + {I_{{L\; 9},i}\cos \; \omega_{10,i}t}} \right\rbrack} \approx 0}}}} & (29) \\ {\begin{matrix} {v_{{Cr},i} = {v_{S,i} - {\left( {v_{S,i} - V_{{C\; 1},i}} \right)\cos \; \omega_{10,i}t} + {Z_{{o\; 10},i}I_{{L\; 9},i}\sin \; \omega_{10,i}t}}} \\ {{\approx {{\frac{i_{S,i}}{C_{r,i} + C_{{S\; 4},i}}t} + V_{{C\; 1},i}}},} \end{matrix}{v_{{S\; 2},i} = {v_{{S\; 3},i} = 0}}} & (30) \end{matrix}$

where I_(L9,i) is the input current at t₉, ω_(10,i)=ω_(4,i) and Z_(o10,i)=Z_(o4,i).

The duration of this stage, Δt_(10,i)=t₁₀−t₉, is obtained by solving (30) that

v _(P,i) −v _(S,i)=−(v _(S,i) −V _(C1,i))cos ω_(10,i) Δt _(10,i) +Z _(o10,i) I _(L9,i) sin ω_(10,i) Δt_(10,i)   (31)

11) Stage 11 [t₁₀-t₁₁] [FIG. 5( k)]: Resonant Stage II

S_(3,i) starts conducting. C_(r,i), L_(r,i), S_(2,i), S_(3,i), the output capacitances of S_(1,i) and S_(4,i), C_(S1,i) and C_(S4,i), and the leakage inductance of the transformer L_(l,i) form a resonant circuit. S_(Au,i) will be turned off with zero current and zero voltage, when the diode D_(Au,i) conducts. This stage ends when D_(Au,i) turns off with zero current.

$\begin{matrix} {{{i_{S,i} = {{\frac{\left( {v_{S,i} - v_{P,i}} \right)C_{r,i}L_{l,i}^{2}}{\left( {L_{i} + L_{l,i}} \right)\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{11,i}t} + {\frac{L_{l,i}I_{{L\; 10},i}}{L_{i} + L_{l,i}}\cos \; \omega_{11,i}t} - {\frac{v_{P,i} - v_{S,i}}{L_{i} + L_{l,i}}t} + \frac{L_{i}I_{{L\; 10},i}}{L_{i} + L_{l,i}}}},\mspace{20mu} {i_{{Cr},i} = {{\frac{\left( {v_{S,i} - v_{P,i}} \right)L_{l,i}C_{r,i}}{\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{11,i}t} + {I_{{L\; 10},i}\cos \; \omega_{11,i}t}}},{i_{{S\; 2},i} = {i_{{S\; 3},i} = {{\frac{\left( {v_{P,i} - v_{S,i}} \right)C_{r,i}L_{i}L_{l,i}}{\left( {L_{i} + L_{l,i}} \right)\sqrt{C_{r,i}{\Phi_{i}\left( {L_{i} + L_{l,i}} \right)}}}\sin \; \omega_{11,i}t} - {\frac{L_{i}I_{{L\; 10},i}}{L_{i} + L_{l,i}}\cos \; \omega_{11,i}t} - {\frac{v_{P,i} - v_{S,i}}{L_{i} + L_{l,i}}t} + \frac{L_{i}I_{{L\; 10},i}}{L_{i} + L_{l,i}}}}},\mspace{20mu} {i_{{S\; 1},i} = {i_{{S\; 4},i} = 0}}}\mspace{11mu}} & (32) \\ {{v_{{Cr},i} = {{I_{{L\; 10},i}Z_{{o\; 10},i}\sin \; \omega_{11,i}t} - {\frac{\left( {v_{S,i} - v_{P,i}} \right)L_{l,i}}{L_{i} + L_{l,i}}\cos \; \omega_{11,t}t} + \frac{{L_{i}v_{P,i}} + {L_{l,i}v_{S,i}}}{L_{i} + L_{l,i}}}},\mspace{20mu} {v_{{S\; 2},i} = {v_{{S\; 3},i} = 0}}} & (33) \end{matrix}$

where I_(L10,i) is the input current at t₁₀, ω_(11,i)=ω_(5,j), Z_(o11,i)=Z_(o5,i), and Φ_(i)=L_(r,i) L_(i)+L_(r,i) L_(l,i)+L_(i) L_(l,i).

The duration of this stage Δt_(11,i)=t_(11−t) ₁₀ is equal to Δt_(5,i).

12) Stage 12 [t₁₁-t₁₂] [FIG. 5( l)]: Inductor Discharging Stage

The energy stored in L_(i) releases to the output together with the input at a relatively constant current. This stage ends when S_(1,i) is turned on with zero current.

$\begin{matrix} {{i_{S,i} = {{\frac{v_{S,i} - v_{P,i}}{L_{i}}t} + I_{{L\; 11},i}}},{i_{{Cr},i} = 0},{i_{{S\; 2},i} = {i_{{S\; 3},i} = {{\frac{v_{S,i} - v_{P,i}}{L_{i}}t} + I_{{L\; 11},i}}}},{i_{{S\; 1},i} = {i_{{S\; 4},i} = 0}}} & (34) \\ {{v_{{Cr},i} = V_{{C\; 0},i}},{v_{{S\; 2},i} = {v_{{S\; 3},i} = 0}}} & (35) \end{matrix}$

where I_(L11,i) is the input current at t₁₁.

The duration of this stage Δt_(12,i)=t₁₂−t₁₁ is equal to Δt_(6,i). This completes a cycle of operation.

For the sake of simplicity in the following analysis of steady-state conditions, the input inductor current i_(S,i) is assumed to be constant throughout the whole period. This is valid because the ripple current is very small, as compared with the average of i_(S,i,).

A. Voltage Conversion Ratio

The voltage conversion ratio is derived by considering the current i_(o,i) on the primary side of the transformer. FIG. 7 shows the waveform of i_(o,i). The electric energy is transferred from the primary side to the secondary in three main intervals in one-half of the switching period. Consider the first half cycle from t₀ to t₆. The intervals are as follows:

-   1) Stage 5—Based on (13), i_(o,i) can be expressed as

i _(o,i) =i _(S,i) −i _(S,i) cos ω_(5,i) t   (36)

The duration of this interval Δt₅ is shown in (15).Δ

-   2) Stage 6—i_(o,i)=i_(S,i). The duration of this interval Δt_(6,i)     is shown in (20). -   3) Stage 7—i_(o,i) decreases linearly, as expressed in (22),

$\begin{matrix} {i_{o,i} = {{{- \frac{v_{P,i}}{L_{l,i}}}t} + i_{S,i}}} & (37) \end{matrix}$

The duration of this interval Δt_(7,i) is determined by solving (37) for i_(o,i)=0. Thus,

$\begin{matrix} {{\Delta \; t_{7,i}} = {{t_{7} - t_{6}} = \frac{L_{l,i}i_{S,i}}{v_{P,i}}}} & (38) \end{matrix}$

Thus, the area under i_(o,i) is equal to

$\begin{matrix} \begin{matrix} {A_{P,i} = {{\int_{0}^{\Delta \; t_{5,i}}{\left\lbrack {i_{S,i} - {i_{S,i}\cos \; \omega_{5,i}t}} \right\rbrack {t}}} +}} \\ {{{\int_{0}^{\Delta \; t_{6,i}}{i_{S,i}{t}}} + {\int_{0}^{\Delta \; t_{7,i}}{\left( {{{- \frac{v_{P,i}}{L_{l,i}}}t} + i_{S,i}} \right){t}}}}} \\ {= {{i_{S,i}\left( {\theta_{i}\frac{T_{S}}{2}} \right)} + {i_{S,i}\sqrt{C_{r,i}\left( {L_{r,i} + L_{l,i}} \right)}} + {\frac{1}{2}\left( \frac{L_{l,i}i_{S,i}^{2}}{v_{P,i}} \right)}}} \end{matrix} & (39) \end{matrix}$

The average output current I_(o,i) of the module is equal to

$\begin{matrix} \begin{matrix} {I_{o,i} = {\frac{2}{T_{S}}\frac{n_{i}}{n_{o}}A_{P}}} \\ {= {i_{S,i}{\frac{n_{i}}{n_{o}}\left\lbrack {\theta_{i} + {\frac{2}{T_{S}}\sqrt{C_{r,i}\left( {L_{r,i} + L_{l,i}} \right)}} + {\frac{1}{T_{S}}\left( \frac{L_{l,i}i_{S,i}}{\frac{n_{i}}{n_{o}}V_{Out}} \right)}} \right\rbrack}}} \end{matrix} & (40) \end{matrix}$

For N modules, the total output current I_(o) is equal to the sum of all module outputs. Thus,

$\begin{matrix} \begin{matrix} {I_{o} = {\sum\limits_{i = 1}^{N}I_{o,i}}} \\ {= {\sum\limits_{i = 1}^{N}{i_{S,i}{\frac{n_{i}}{n_{o}}\left\lbrack {\theta_{i} + {\frac{2}{T_{S}}\sqrt{C_{r,i}\left( {L_{r,i} + L_{l,i}} \right)}} + {\frac{1}{T_{S}}\left( \frac{L_{l,i}i_{S,i}}{\frac{n_{i}}{n_{o}}V_{Out}} \right)}} \right\rbrack}}}} \end{matrix} & (41) \end{matrix}$

By using the conservation of energy, the voltage conversion ratio can be shown to be

$\begin{matrix} \begin{matrix} {{i_{S,i}V_{S,i}} = {i_{o}V_{Out}}} \\ \left. \Rightarrow \frac{V_{Out}}{V_{S,i}} \right. \\ {= {\frac{n_{o}}{n_{i}}\frac{1 - \frac{L_{l,i}i_{S,i}}{T_{S}v_{S,i}}}{\theta_{i} + {\frac{2}{T_{S}}\sqrt{C_{r,i}\left( {L_{r,i} + L_{l,i}} \right)}}}}} \end{matrix} & (42) \end{matrix}$

Thus, the output voltage can be controlled by adjusting the value of θ_(i), which is the phase shift between the two switch pairs.

B. Selection of L_(l,i), C_(r,i), and L_(r,i)

1. L_(l,i)

L_(l,i) is used to achieve ZCS of the two high side switches. The maximum value of L_(l,i), L_(l(max),i), determines the critical ZCS condition in stages 1 or 7 at the maximum power P_(max,i). By using (2), it can be shown that

$\begin{matrix} {L_{{l{(\max)}},i} = \frac{n_{i}V_{out}v_{s,i}\Delta \; t_{1,i}}{n_{o}P_{\max,i}}} & (43) \end{matrix}$

where i_(S(max),i) is the maximum value of i_(S,i).

The duration of Δt_(1,i) is an adjustable variable in the circuit implementation. It is practically determined by the controller integrated circuit [10]. Thus, the output power can be increased by increasing Δt_(1,i)—the overlap time of the two high side switches.

2. C_(r,i) and L_(r,i)

The minimum value of C_(r,i) is determined by ensuring the ZCS conditions for the two low side switches in stages 3 or 9 at the minimum power P_(min,i). The durations Δt_(3,i) or Δt_(9,i) are equal to π/2ω_(3,i). Thus, by using (7),

Δt _(3,i)=π/2ω_(3,i)=π/2√{square root over (C _(r,i) L _(r,i))}  (44)

By assuming I_(L4,i)=i_(S(min),i) and using (16) and (6),

$\begin{matrix} \begin{matrix} {{- \frac{V_{{C\; 0},i}}{Z_{{o\; 3},i}}} = i_{S,{(\min)},i}} \\ \left. \Rightarrow C_{r,i} \right. \\ {= \left\lbrack \frac{\left( {\sqrt{L_{l,i} + L_{r,i}} - \sqrt{L_{r,i}}} \right)i_{{S{(\min)}},i}}{v_{P,i}} \right\rbrack^{2}} \end{matrix} & (45) \end{matrix}$

where i_(S(min),i) is the value of i_(S,i) at the minimum input power. By solving (44) and (45) for C_(r,i) and L_(r,i), it can be shown that

$\begin{matrix} {{C_{r,i} = {\frac{n_{o}P_{\min}}{n_{i}V_{out}v_{s,i}}\left\lbrack {{L_{l,i}\left( \frac{n_{o}P_{\min}}{n_{i}V_{out}v_{s,i}} \right)} - {2\left( {\Delta \; t_{3,i}\frac{2}{\pi}} \right)}} \right\rbrack}}{and}} & (46) \\ {L_{r,i} = \frac{\left( {\frac{n_{i}V_{out}v_{s,i}}{n_{o}P_{\min}}\Delta \; t_{3,i}\frac{2}{\pi}} \right)^{2}}{L_{l,i} - {2\frac{n_{i}V_{out}v_{s,i}}{n_{o}P_{\min}}\left( {\Delta \; t_{3,i}\frac{2}{\pi}} \right)}}} & (47) \end{matrix}$

According to (45), in order to ensure a positive value of C_(r,i), the value of L_(l,i) has to satisfy the following constraint of

$\begin{matrix} {{L_{l,i} > L_{{l{(\min)}},i}} = {2\frac{n_{i}V_{out}v_{s,i}}{n_{o}P_{\min}}\left( {\Delta \; t_{3,i}\frac{2}{\pi}} \right)}} & (48) \end{matrix}$

As shown in (46), the value of C_(r,i) increases with the increase in the value of L_(l,i) for a given power level.

C. Selection of the Transformer Turns-Ratio n_(o)/n_(i)

The transformer turns ratio is selected by considering the converter at the minimum power condition P_(min). By rearranging (45),

$\begin{matrix} {P_{\min,i} = {{v_{S,i}i_{{S{(\min)}},i}} = \frac{v_{S,i}v_{P,i}}{Z_{{o\; 5},i} - Z_{{o\; 3},i}}}} & (49) \end{matrix}$

Based on (42), the output power decreases as θ_(i) increases. By substituting (49) into (42) and eliminating i_(S(min),i), it can be shown that

$\begin{matrix} {{\frac{n_{i}}{n_{o}}\left\lbrack {\theta_{i{(\max)}} + {\frac{2}{T_{S}}\sqrt{C_{r,i}\left( {L_{r,i} + L_{l,i}} \right)}} + {\frac{1}{T_{S}}\left( \frac{L_{l,i}}{Z_{{o\; 5},i} - Z_{{o\; 3},i}} \right)}} \right\rbrack} = \frac{v_{S,i}}{V_{Out}}} & (50) \end{matrix}$

where θ_(i(max)) is the maximum value of θ_(i) and is derived as follows.

θ_(i(max)) occurs when Δt_(4,i) is close to zero. Thus, according to FIG. 6,

$\begin{matrix} {{\theta_{i{(\max)}}\frac{T_{S}}{2}} = {\frac{T_{S}}{2} - {\Delta \; t_{1,i}} - {\Delta \; t_{2,i}} - {\Delta \; t_{3,i}}}} & (51) \end{matrix}$

Δt_(1,i), Δt_(2,i), and Δt_(3,i) are set by the external components in the phase-shift controller, such as UC3875. By substituting (51) into (50), the maximum turns ratio μ_(max) is equal to

$\begin{matrix} {{{\mu_{\max} = \frac{n_{o}}{n_{i}}}}_{\max} = {\eta \; {\frac{2}{T_{S}}\left\lbrack {\theta_{\max,i} + \frac{1}{\omega_{5,i}} + \frac{L_{l,i}}{2 \cdot \left( {Z_{{o\; 5},i} - Z_{{o\; 3},i}} \right)}} \right\rbrack}}} & (52) \end{matrix}$

FIG. 8 illustrates one example of a suitable control circuit 800 for the auxiliary circuit 220 shown in FIG. 3. As shown in FIG. 8, the control circuit 800 includes a NOR gate U_(4,i) for monitoring the overlapping period of switching gate signals v_(GS3,i) and v_(GS4,i). An LC filter network R_(1,i) and C_(1,i), together with a positive DC voltage V_(DC3,i) and a comparator U_(3,i), is used to determine the turn-on time of the auxiliary switch S_(Au,i) during the overlapping period.

A current transformer T_(C,i) with turns-ratio 1:n is connected in series in the auxiliary circuit, where n is a large integer such as 140. The current transformer is used to step down the current flowing through the auxiliary circuit. Resistor R_(S,i) is a current sense resistor which is connected in series with the secondary winding of the current transformer.

The turn-on signal provided by the comparator U_(3,i) cannot be maintained after the overlapping period. Therefore, comparators U_(1,i), U_(2,i), an RS latch U_(5,i), and an OR gate U_(6,i) are provided to maintain the turn-on signal v_(GSAu,i) for the auxiliary switch S_(Au,i) throughout Stages 3, 4 and 5.

The comparator U_(1,i), which receives a positive DC voltage V_(DC1,i), latches the output of the RS latch U_(5,i) to Logic “1” when it detects the rising voltage across the current sense resistor R_(S,i) during Stage 3. The OR gate U_(6,i) combines the Logic “1” signals from the comparator U_(3,i) and the RS latch U_(5,i) to maintain the turn-on signal v_(GSAu,i) high. When the signal from the comparator U_(3,i) changes to a Logic “0” during Stage 4, the output of the RS latch U_(5,i) maintains the turn-on signal v_(GSAu,i) high to keep the auxiliary switch S_(Au,i) on.

The comparator U_(2,i), which receives a negative DC voltage −V_(DC2,i), is used to remove the gate signal v_(GSAu,i) when the body diode of the auxiliary switch S_(Au,i) conducts at Stage 5. When the comparator U_(2,i) detects the negative voltage across the current sense resistor R_(S,i), it resets the RS-latch U_(5,i) to Logic “0,” which causes the output of the OR gate to become low so that the auxiliary switch S_(Au,i) is turned off when its body diode is conducting. 

1. A multi-input dc/dc converter comprising: a plurality of input modules, each input module including at least one input terminal for coupling to a dc power source and a full bridge converter having a plurality of main switches; a transformer having a plurality of primary windings and at least one secondary winding, each primary winding coupled to one of the full bridge converters; at least one output terminal for supplying dc power to a load; and a rectifier circuit coupled between the secondary winding of the transformer and said output terminal; each input module further including an auxiliary circuit coupled to the full bridge converter for achieving substantially zero-current-switching of the main switches during turn-on and turn-off of the main switches.
 2. The converter of claim 1 wherein the auxiliary circuit of each input module is coupled to an input side of the full bridge converter.
 3. The converter of claim 1 wherein each input module further includes an input choke coupled between said input terminal and the full bridge converter.
 4. The converter of claim 3 wherein each auxiliary circuit includes an auxiliary switch having a body diode that inhibits current flow from the input choke when the auxiliary switch is off.
 5. The converter of claim 1 wherein each full bridge converter is a phase-shift-controlled full bridge converter.
 6. The converter of claim 1 wherein each auxiliary circuit includes a switch, a capacitance, and an inductance.
 7. The converter of claim 6 wherein said switch, said capacitance, and said inductance are coupled in series in each input module.
 8. The converter of claim 1 wherein each auxiliary circuit includes an auxiliary switch and is configured to achieve substantially zero-current-switching of the auxiliary switch during turn-on and substantially zero-voltage-switching during turn-off.
 9. The converter of claim 8 wherein each auxiliary circuit includes a control circuit for controlling said auxiliary circuit switch.
 10. The converter of claim 9 wherein each auxiliary circuit includes a current transformer coupled to said control circuit.
 11. The converter of claim 1 wherein each full bridge converter includes four main switches each configured for unidirectional operation.
 12. A multi-input DC/DC converter comprising: a plurality of input modules, each input module including at least one input terminal for coupling to a dc power source, a full bridge converter having a plurality of main switches, and means for achieving substantially zero-current-switching of the main switches during turn-on and turn-off of the main switches; a transformer having a plurality of primary windings and at least one secondary winding, each primary winding coupled to one of the full bridge converters; at least one output terminal for supplying dc power to a load; and a rectifier circuit coupled between the secondary winding of the transformer and said output terminal.
 13. The converter of claim 12 wherein said means for achieving substantially zero-current-switching in each input module includes a switch, a capacitance, and an inductance coupled to an input side of the full bridge converter.
 14. A DC/DC converter comprising: at least one input terminal for coupling to a dc power source; a full bridge converter having a plurality of main switches; a transformer having at least one primary winding at least one secondary winding, the primary winding coupled to said full bridge converter; at least one output terminal for supplying dc power to a load; a rectifier circuit coupled between the secondary winding of the transformer and said output terminal; and an auxiliary circuit coupled between the input terminal and the full bridge converter for achieving substantially zero-current-switching of the main switches during turn-on and turn-off of the main switches.
 15. The converter of claim 14 further comprising an input choke coupled between said input terminal and the auxiliary circuit.
 16. The converter of claim 15 wherein the auxiliary circuit includes an auxiliary switch having a body diode that inhibits current flow from the input choke when the auxiliary switch is off.
 17. The converter of claim 14 wherein the full bridge converter is a phase-shift-controlled full bridge converter.
 18. The converter of claim 14 wherein the auxiliary circuit includes a switch, a capacitance, and an inductance.
 19. The converter of claim 18 wherein said switch, said capacitance, and said inductance are coupled in series.
 20. The converter of claim 19 wherein the auxiliary circuit is configured to achieve substantially zero-current-switching of the auxiliary switch during turn-on and turn-off of the auxiliary switch.
 21. The converter of claim 14 wherein the full bridge converter includes four main switches each configured for unidirectional operation. 